Express.js Middleware: A Secret Ingredient to Build Scalable Web Applications

Quick Overview: Express.js middleware is a vital component for building scalable web applications. This blog explains what middleware is and how it works in the context of Express.js. In addition, you will learn the functions, types of middleware, and how to write middleware. By the end of the blog, readers will have a solid understanding of how Express.js middleware can enhance the functionality and scalability of their web applications.

Express.js, a feature-rich web application framework used to build high-performing apps quickly and simply. But what makes Express.js a highly potential framework is its middleware function. It is a powerful feature that allows you to enhance the functionality of your Express.js application. Basically, middleware acts as a bridge between databases, tools, and technology to let the users integrate into a single system and provide a unified outcome.

Moreover, Express.js is mainly used by companies with 1M-10M dollars in revenue. And in the software frameworks category, Express.js has a market share of about 0.7%.

In today’s article, we will explore the concept of middleware in Express.js, its fundamentals, functions, working cycle, and its utilization to build a next-gen web application.

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What is Middleware?

Middleware refers to software that lies between the application and the server to communicate with each other. Basically, it intercepts requests and responses, and performs additional practices before passing them on to the final route handler. You can utilize it for various purposes, including authentication, logging, error handling, and data parsing.

As the name signifies, it works in the middle to deliver services to applications. Let's get a clear view with the illustration shown below:

Middleware is also accessible to change the requests and response objects, and can also terminate the response cycle, if necessary. This simplicity and flexibility make it a vital function of Express.js framework.

What is Express.js Middleware?

Simply put, Middleware in Express.js is a function that is accessible to the request, response objects and the next middleware function. It can execute any code, terminate the request-response cycle, make needed changes in req and res objects, and detect errors.

In Express.js, we can conclude middleware in three arguments, i.e.; the request object (req), the response object (res), and by choice the next () middleware function.

Since installing Express.js has confined functionalities of its own, it utilizes multiple middleware functions by using the next () function to simplify and improve web application operation. However, developers can also write their own Express Custom Middleware for their Express.js web application.

For a comprehensive understanding, let’s examine a practical express middleware example:

A user visits your website and logs in with some credentials. Now, the request element has been sent to your web server. Meanwhile, you are required to verify the user also his/her login database, such as a particular <user> land up at the given <time> via the <IP> and <used device> before preparing or sending the response object to the browser.

Furthermore, you are also required to check your website’s page that the user searched for the last time and modify the request object accordingly.

And how do you do this entire process? With one and only Middleware Function!

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Functions of Express.js Middleware

We can use middleware functions to perform various tasks in an Express.js application:

1. Modify the request-response cycle by making required additions.
2. We can also terminate or reject the request-response cycle.
3. Middleware is also accessible to transfer the request to the next middleware function.
4. Provides a modular way to execute codes and embrace reusable functionality.
5. We can chain multiple middleware together.

Different Types of Express.js Middleware

You can use the following types of middleware in an Express.js application:

1. Application-level middleware

It runs for all routes in an application. When a user receives an authentication request program, the middleware moves toward authentication code logic to approve the authentication. And use the next () function to call the rest of the route. If authentication fails, the middleware will show an error and the cycle will not respond to the next route.

2. Router-level middleware

This middleware function works the same way as the application level but can generate or limit an instance using Express.route() function or router.use() function. It runs for all routes in a router object only.

3. Built-in middleware

This function helps Express to act as a module. You can utilize the Express.json function to compute and add payloads and the Express. static function to act as application static assets.

4. Error-handling middleware

This middleware function helps to handle errors in the req-res cycle. It defines the middleware function in the same way with three arguments.

5. Third-party middleware

The user needs to install the node.js technology and load it to the application level at the router level to let the third-party middleware function work properly. ExpressJS supports lots of third-party middleware functions, such as

• The bodyParser function is used to parse the requests with payloads and can be installed using npm install.
• The cookieParser function parses the cookie header by cookie names and can be mounted by the npm install function.

How to Write and Create Express Middleware?

Middleware functions are written in chain format to execute a code order with the next () function that calls the next express middleware function if needed as shown in the below image:

The basic Express middleware syntax is

Now, let’s learn how to create and execute middleware in Express.js with an example!

Here are three middleware functions that we can create and add to the Express.js application mentioned below:

1. Middleware function myLogger

This function prints LOGGED when a request is made on the application. Then on, the middleware function will get assigned to the myLogger variable by using the following command:

Whenever the application receives a request, it will print 'LOGGED' to the terminal. Note that the middleware functions will execute in the order they loaded as myLogger and will just print a message and pass the request to the next middleware function in the line with the next () function.

2. Middleware function requestTime

requestTime function displays the time stamp of the HTTP request. First, we will create the requestTime middleware function by adding it to the request object by using the following command:

By utilizing the requestTime function, the application will execute the requestTime middleware function and display the timestamp of the request in the browser.

3. Middleware Function validateCookies

This middleware function will validate the incoming cookies and send a 400 error as a response if they have invalid cookies. By following the code, you can validate cookies with an external async service in express JS.

In this code, we used cookie-parser middleware to pass incoming cookies off the req object and move them to the cookieValidator function. The cookieValidator middleware function will respond automatically to trigger the error handler.

The possibilities with the middleware function in ExpressJS are endless as you have full access to the req object, res object, and next () middleware function in the cycle.

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ConclusionBy leveraging middleware functions, you can seamlessly include additional features and make a web application experience more secure and functional. With its capability to register multi-middleware functions and error-handling approach, Express.js (a renowned Node.js framework) empowers a modular and flexible approach in web application development.If you are facing any trouble to utilize middleware functions and in any other backend technologies, contact us anytime to get a free expert consultation from Radixweb Experts.Radixweb is a globally trusted Node.js development company, offering exceptional software development, web, and app development solutions since 2001. Our 650+ certified tech talents always strive to accelerate innovation in the global technology landscape.So, without any further ado, talk to our experts and hire Node.js developers to be at the forefront of developing a robust web application.Stay tuned for more detailed insights and the latest tech topics.